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winter ventilation mistakes

In early November I had data loggers placed in production facilities in west central Minnesota to collect producer data for an upcoming ventilation workshop. In addition to logging temperatures every 2 minutes for a 7 day period we collected information on the ventilation system, including inlets, fans, controllers and controller settings.

We’ve been putting data loggers in barns for over 10 years for workshops and the results always come back documenting both barns with correctly sized and operated components and barns with lots of mistakes in both design and management. With winter weather upon us let’s talk about the common mistakes I saw in this and other data sets that result in higher than necessary fuel bills and ventilation systems that don’t behave as expected.
Getting minimum ventilation correct is the key to keeping propane expenses in check. Both in the data from these loggers and from production sites I’ve been at this fall, a common installation error is to have the fan staging in the controller incorrect. In the sites I’m thinking about (5-600 head rooms), there have been 2-24” fans on stage 1 and only 1-24” fan on stage 2 with the curtain opening on stage 3.

It is just about impossible to get 2-24” fans slowed down enough in cold weather to get anywhere near recommended minimum ventilation rates. Most 24” fans are in the range of 6000 cfm. With 2 fans this means you have 12,000 cfm capacity in stage 1. Assuming you set the controller correctly to get 50% of the cfm as the minimum, this is still 6000 cfm. With 5-600 head rooms, this is 10-12 cfm/pig minimum. This minimum compares to the recommended 5 cfm for pigs at 50-80 lb.

In cold weather this difference represents propane since you remove 1.08 btu of heat per cfm per degree F difference in outside and inside air temperature. If it is 0F outside and 68F in the animal space, this is 68F difference in temperature. At 5 cfm/pig over ventilation because of a fan staging error, this becomes (5 cfm x 1.08 btu/hr/cfm/F x 68F) 357 btu of heat per hour that is wasted. With 500 pigs in a room it becomes 178,500 btu. Given there are 92,000 btu/gal of propane, this is now 2 gal/hr of wasted propane.

The simple solution in this case is to reverse the wiring in the controller for stages 1 and 2.

Note that earlier I talked about controller settings. In every instance where we’ve put data loggers in barns we’ve gotten back data from at least 1 site where the heating stage off setting is at set point. In almost every controller this results in the variable speed stage 1 fan ramping up to remove heat after the furnace turns off. In cold weather when the furnaces are operating, the room temperature should never get closer than about 0.5F below the temperature at which the fan(s) on stage 1 increase their ventilation rate.

You didn’t say anything about a controller. I assume the 10″ fan is the stage 1 variable fan (about 900-1000 cfm at full speed which is 45 cfm/crate). The recommended minimum for farrowing rooms to control moisture is 20 cfm/crate and I usually try for about 25 cfm when sizing the stage 1 fan.

Each of the 24″ fans should be a separate stage. If stage 2 is variable, my controller settings with a 74F set point would be:
Stage 1 – 2 bandwidth – full speed at 76
Stage 2 – 1.5 band width – turns on at 77 and off at 76.5 (Many times with more appropriately sized fans I turn off at 76 but with such a big fan as stage 2 you will tend to get wild temperature swings even after the fan turns off as the incoming air disperses in the room). Assuming 6000 cfm for the 24″ fan, this is 273 cfm/crate – quite a jump from the 45 in stage 1)
Stage 3 – normally turns on at 1F above stage 2 full speed but with such an oversizing of fans I would do 1.5 or even 2.

Some of our facilities have taken off all fan hoods on the stage 1 & 2 variable fans. Their reasoning for this is that it has corroded the bottom tin below these fans. My concern with this is that we now have no protection from the wind for these variable speed fans. I think this will create many problems.

Yes, there is the possibility of wind impacting the fans more without wind hoods. this is one of many reasons variable speed fans should never operate at less that 50% of the motor’s rated rpm which equates to approximately 40% of the cfm. If you have 2 variable fans on stage 1 and operate the minimum less than 50%, consider unplugging one of the fans and operate the remaining fan at a higher speed which will be more capable of competing with wind.

Wind hoods don’t do as much to protect fans from winds as you might think. Winds pressurize the entire side of the building and even with hoods installed impact fan performance. Hoods decrease fan performance in general as it takes energy to divert the direction of the exhaust air. With the hoods removed you can easily see a 10-15% increase in fan cfm.

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